Rotobrush 3.0 when pre-compose a black outline appear

Hello, I was wondering if you could help me with a problem I'm currently facing in After Effects.

I used the Rotobrush 3.0 on a clip and everything looked fine within the layer. After that, I pre-composed the rotobrushed layer. However, when I view the pre-comp in my main composition, a black outline appears around the subject, when I open the pre-comp by itself, the black outline doesn't appear at all.

Do you happen to know what might be causing this, and how I can remove the outline while keeping the transparency intact in the main comp?